The present invention relates to the manufacture of electronic devices. More particularly, the invention provides a device for planarizing a film of material of an article such as a semiconductor wafer. In an exemplary embodiment, the present invention provides an improved substrate support for the manufacture of semiconductor integrated circuits. However, it will be recognized that the invention has a wider range of applicability; it can also be applied to flat panel displays, hard disks, raw wafers, MEMS wafers, and other objects that require a high degree of planarity.
The fabrication of integrated circuit devices often begins by producing semiconductor wafers cut from an ingot of single crystal silicon which is formed by pulling a seed from a silicon melt rotating in a crucible. The ingot is then sliced into individual wafers using a diamond cutting blade. Following the cutting operation, at least one surface (process surface) of the wafer is polished to a relatively flat, scratch-free surface. The polished surface area of the wafer is first subdivided into a plurality of die locations at which integrated circuits (IC) are subsequently formed. A series of wafer masking and processing steps are used to fabricate each IC. Thereafter, the individual dice are cut or scribed from the wafer and individually packaged and tested to complete the device manufacture process.
During IC manufacturing, the various masking and processing steps typically result in the formation of topographical irregularities on the wafer surface. For example, topographical surface irregularities are created after metallization, which includes a sequence of blanketing the wafer surface with a conductive metal layer and then etching away unwanted portions of the blanket metal layer to form a metallization interconnect pattern on each IC. This problem is exacerbated by the use of multilevel interconnects.
A common surface irregularity in a semiconductor wafer is known as a step. A step is the resulting height differential between the metal interconnect and the wafer surface where the metal has been removed. A typical VLSI chip on which a first metallization layer has been defined may contain several million steps, and the whole wafer may contain several hundred ICs.
Consequently, maintaining wafer surface planarity during fabrication is important. Photolithographic processes are typically pushed close to the limit of resolution in order to create maximum circuit density. Typical device geometries call for line widths on the order of 0.5 xcexcm. Since these geometries are photolithographically produced, it is important that the wafer surface be highly planar in order to accurately focus the illumination radiation at a single plane of focus to achieve precise imaging over the entire surface of the wafer. A wafer surface that is not sufficiently planar, will result in structures that are poorly defined, with the circuits either being nonfunctional or, at best, exhibiting less than optimum performance. To alleviate these problems, the wafer is xe2x80x9cplanarizedxe2x80x9d at various points in the process to minimize non-planar topography and its adverse effects. As additional levels are added to multilevel-interconnection schemes and circuit features are scaled to submicron dimensions, the required degree of planarization increases. As circuit dimensions are reduced, interconnect levels must be globally planarized to produce a reliable, high density device. Planarization can be implemented in either the conductor or the dielectric layers.
In order to achieve the degree of planarity required to produce high density integrated circuits, chemical-mechanical planarization processes (xe2x80x9cCMPxe2x80x9d) are being employed with increasing frequency. A conventional rotational CMP apparatus includes a wafer carrier for holding a semiconductor wafer. A soft, resilient pad is typically placed between the wafer carrier and the wafer, and the wafer is generally held against the resilient pad by a partial vacuum. The wafer carrier is designed to be continuously rotated by a drive motor. In addition, the wafer carrier typically is also designed for transverse movement. The rotational and transverse movement is intended to reduce variability in material removal rates over the surface of the wafer. The apparatus further includes a rotating platen on which is mounted a polishing pad. The platen is relatively large in comparison to the wafer, so that during the CMP process, the wafer may be moved across the surface of the polishing pad by the wafer carrier. A polishing slurry containing chemically-reactive solution, in which are suspended abrasive particles, is deposited through a supply tube onto the surface of the polishing pad.
CMP is advantageous because it can be performed in one step, in contrast to past planarization techniques which are complex, involving multiple steps. Moreover, CMP has been demonstrated to maintain high material removal rates of high surface features and low removal rates of low surface features, thus allowing for uniform planarization. CMP can also be used to remove different layers of material and various surface defects. CMP thus can improve the quality and reliability of the ICs formed on the wafer.
Chemical-mechanical planarization is a well developed planarization technique. The underlying chemistry and physics of the method is understood. However, it is commonly accepted that it still remains very difficult to obtain smooth results near the center of the wafer. The result is a planarized wafer whose center region may or may not be suitable for subsequent processing. Sometimes, therefore, it is not possible to fully utilize the entire surface of the wafer. This reduces yield and subsequently increases the per-chip manufacturing cost. Ultimately, the consumer suffers from higher prices.
It is therefore desirable to improve the useful surface of a semiconductor wafer to increase chip yield. What is needed is an improvement of the CMP technique to improve the degree of global planarity that can be achieved using CMP.
The present invention achieves these benefits in the context of known process technology and known techniques in the art. The present invention provides an improved planarization apparatus for chemical mechanical planarization (CMP). Specifically, the present invention provides an improved planarization apparatus that provides multi-action CMP, such as orbital and spin action, to achieve uniformity during planarization.
In an exemplary embodiment, an apparatus of the invention allows both orbital and pure spin motion of a polishing head that holds a polishing pad which is smaller in size than the wafer for planarizing the wafer. An orbit housing is held in place by bearings and driven directly by a motor or through a belt or a gear. The orbit housing has an eccentric or offset hole which supports a shaft with bearings. The shaft is connected to the polishing head. An external tooth gear or friction drive is attached to the shaft and mates with an internal tooth gear or friction drive. The internal tooth gear is a ring gear supported by another bearing concentric with the outer orbit housing bearings, and is driven by a second direct motor, or a second gear or belt drive. By controlling the relative speeds of the two motors, the polishing head can be made to spin only (while holding the orbit motor stationary), to spin and orbit (i.e., to precess), or to orbit only (by controlling the relative motions of the two motors so that the polishing pad does not spin relative to the wafer).
The inventors have discovered that improved uniformity of planarization can be achieved by polishing the center of the wafer by predominately orbital motion and polishing the edge of the wafer by predominately spin motion. The inventors have also found that uniformity is improved if, during combined orbital motion and rotation of the wafer, the ratio of the greater of the orbiting speed and the wafer rotational speed to the lesser of the two is a non-integer.
In accordance with an aspect of the present invention, a chemical-mechanical planarization apparatus for planarizing an object comprises a shaft having a shaft axis and being connected with a polishing head which is coupled to a polishing pad. The polishing pad has a smaller diameter than the object to be planarized. The shaft is rotatable to spin the polishing head and polishing pad around the shaft axis. An orbit housing has, spaced from an orbital axis, an eccentric hole through which the shaft is rotatably disposed. The orbit housing is rotatable to orbit the shaft, the polishing head, and the polishing pad around the orbital axis. In some embodiments, the shaft axis is spaced from the orbital axis by an offset which is between about {fraction (1/100)} to about xc2xd the radius of the polishing pad.
In some embodiments, the shaft is connected to an external tooth gear which is rotatably coupled with an internal tooth gear that is configured to be driven in rotation to rotate the external tooth gear to spin the shaft and polishing pad around the shaft axis. A platen is provided for supporting the object to be planarized. The platen is rotatable to rotate the object. The shaft is driven to rotate by a first motor and the orbit housing is driven to rotate by a second motor.
In accordance with another aspect of the invention, a method of planarizing an object by chemical-mechanical planarization using a polishing pad comprises rotating the object relative to an object axis of the object which is perpendicular to a surface of the object to be planarized, and rotating the polishing pad around an orbital axis which is spaced from an axis of the polishing pad. The polishing pad in rotation around the orbital axis is contacted with the surface of the object.
In some embodiments, the object axis is parallel to and spaced from the orbital axis. The object is rotated at an object rotational speed and the polishing pad is rotated around the orbital axis at an orbiting speed. A ratio of the greater of the object rotational speed and the orbiting speed to the lesser of the object rotational speed and the orbiting speed is a non-integer. The orbiting speed may be greater than the object rotational speed.
In some embodiments, the polishing pad is rotated around a pad axis of the polishing pad to spin the polishing pad. The polishing pad is rotated around the pad axis at a spinning speed and is rotated around the orbital axis at an orbiting speed. The orbiting speed is greater than the spinning speed when the polishing pad is contacted with a center region of the surface of the object. In a specific embodiment, the spinning speed is approximately zero when the polishing pad is contacted with the center region. In some embodiments, the spinning speed is greater than the orbiting speed when the polishing pad is contacted with an edge region of the surface of the object. In a specific embodiment, the orbiting speed is approximately zero when the polishing pad is contacted with the edge region.